An active noise control or canceling (ANC) system helps improve the user's listening experience by striving to produce a quieter environment. An “anti-noise” sound wave is produced in such a way that is intended to destructively interfere with or cancel the ambient or background noise sound that would otherwise be heard by the user. In consumer electronics personal listening devices, such as smartphones and portable audio devices such as tablet computers and laptop computers, the listening device often does not have sufficient passive noise attenuation. For instance, a more confortable loose fitting ear bud provides lesser passive ambient noise reduction than a sealed in-ear one. Also, the user is often moving around with a listening device, e.g. walking or jogging. In the case of a smart phone being used in handset mode (against the ear), different users hold the phone differently against their ear, have varied ear anatomy, and tend to move it around during a phone call. All of these user-specific factors may change the acoustic environment or acoustic loading of the listening device in real-time. As a result, attempts are being made to improve the performance of the ANC system in personal listening devices by making the system adaptive. An adaptive filter and an adaptive controller aim to model the different parts of the acoustic environment surrounding the user, or the various acoustic paths leading to the user's eardrum, and to adapt or change the state of the adaptive filter so as to produce an anti-noise signal that better cancels the offending or unwanted noise.
In situations where the noise to be cancelled has transient characteristics, also referred to here as transient disturbances, the adaptive ANC system often loses its bearings, in that it fails to properly drive the adaptive filter. Examples of such “abnormal noise” include for example a police siren, a sudden wind burst, and a scratch of the housing of the personal listening device. This may cause the adaptive system to “diverge” from a solution to the noise cancellation problem, and thereby produce incorrect anti-noise, which of course leads to poor performance (because the noise is now being heard by the user). Transient disturbances are difficult to cancel in the small confines of personal listening devices, due to there being insufficient distance or time lapse between when the disturbance is picked up by a reference microphone and when the anti-noise should be available to cancel it. Moreover, transient disturbances appear suddenly and typically do not last very long, compared to other “normal” ambient or background noise that is long term and essentially periodic.